Preparation of Fe/activated carbon directly from rice husk pyrolytic carbon and its application in catalytic hydroxylation of phenol

Article abstract of DOI:10.1039/c4ra13248c

Rice husk pyrolytic carbon (PC) was pretreated by NaOH solution at 100°C for 5 h to remove SiO₂ and then used to prepare Fe/activated carbon catalyst. The treated sample was impregnated with ferric nitrate solution, and then activated under N₂ atmosphere, obtaining Fe/activated carbon catalysts. The samples were characterized by temperature programed decomposition-mass spectra (TPD-MS), Brauner-Emmett-Teller (BET), inductively coupled plasma atomic emission spectrometry (ICP-AES), scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The specific surface area increased with activation temperature until 750°C. With the increase of iron content, the specific surface area of carbon increased first up to 0.39 mmol g^-1 iron loaded and then decreased. Reactive decomposition of ferric nitrate happened at 120-350°C releasing NO and CO₂. Part of ferric (Fe(III)) species was reduced to ferrous (Fe(II)) species forming Fe₃O₄ at 400-550°C, and metal Fe at 650-750°C. The Fe/activated carbon exhibited high activity and selectivity for phenol hydroxylation.

Full text of DOI:10.1039/c4ra13248c

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PAPER
Preparation of Fe/activated carbon directly from
rice husk pyrolytic carbon and its application in
catalytic hydroxylation of phenol
Cite this: RSC Adv., 2015, 5, 4984
Xian Zhang, Yaxin Li, Guiying Li* and Changwei Hu*
ꢀ
Rice husk pyrolytic carbon (PC) was pretreated by NaOH solution at 100 C for 5 h to remove SiO
2
and then
used to prepare Fe/activated carbon catalyst. The treated sample was impregnated with ferric nitrate
solution, and then activated under N atmosphere, obtaining Fe/activated carbon catalysts. The samples
2
were characterized by temperature programed decomposition-mass spectra (TPD-MS), Brauner–
Emmett–Teller (BET), inductively coupled plasma atomic emission spectrometry (ICP-AES), scanning
electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The
ꢀ
specific surface area increased with activation temperature until 750 C. With the increase of iron
Received 27th October 2014
Accepted 4th December 2014
ꢁ1
content, the specific surface area of carbon increased first up to 0.39 mmol g iron loaded and then
ꢀ
decreased. Reactive decomposition of ferric nitrate happened at 120–350 C releasing NO and CO
2
. Part
DOI: 10.1039/c4ra13248c
ꢀ
of ferric (Fe(III)) species was reduced to ferrous (Fe(II)) species forming Fe
3
O
4
at 400–550 C, and metal
ꢀ
www.rsc.org/advances
Fe at 650–750 C. The Fe/activated carbon exhibited high activity and selectivity for phenol hydroxylation.
phenol oxidation with H
display better activity than other transition metals, although
2 2
O as oxidant. It is revealed that iron
1
. Introduction
3
Rice husk (RH) is an agricultural residue abundantly available other catalysts, mostly transition metals supported on zeolites
in rice-producing countries. The annual worldwide output has and other porous materials, such as TS-1 (ref. 4 and 5) and
6
been estimated to be 80 million tons, of which about a half is Ti-MCM-41, were commonly used. However, their wide indus-
1
generated in China. However, rarely researches concerning its trial applications were limited by the high cost with multi-step
utilization are carried out. If this agricultural residue is not preparation. So many researchers have studied the perfor-
utilized properly, tremendous waste will be produced, causing mance of activated carbon supported Fe species due to its lower
7,8
energy loss and environmental pollution. Rice husk has been cost. Commercially available activated carbon with high
2
used to produce bio oil via pyrolysis, with the production of a surface area was usually used as support to prepare the catalyst
8,9
lot of rice husk powder, pyrolytic carbon (PC), as the by-product. and adsorbent.
Impregnation method was the most
The preparation of activated carbon (AC) from agricultural commonly used for the loading of active components, such as
9,10
residue can not only reduce environmental pollution but also Fe, Cu, etc. AC could be produced from waste materials such
11
reduce the cost of activated carbon production. It has great as rice huck lignite by high temperature treatment. Physical
research value to use PC to prepare activated carbon. This can and chemical activation methods are fundamentally employed
simultaneously provide ways to solve the problem of pollution for the preparation of ACs. Physical activation involves reaction
12
and take full advantage of raw material, that is, the components at high temperatures in steam or carbon dioxide. In chemical
of rice husk are totally used, producing both energy material activation, raw material is mixed with an activator, and the
1
3,14
(
bio-oil) and useful AC.
Functionalized and metal impregnated activated carbons the preparation of AC-supported catalyst, such as magnetic
mixture was subjected to heat treatment.
That is to say, for
15
(
ACs) were promising catalysts and sorbents for various indus- Fe O -activated carbon, two high temperature steps were
3 4
trial applications. For example, iron containing catalysts had included, that is, the preparation of activation carbon involved
high catalytic activity and good effects for removing the harmful high temperature process and the loading of iron also
metal ion in waste water. It was also reported that iron con- encountered high temperature process. If the activation process
taining activated carbon were active and selective catalysts for and iron loading process could be combined in one step, it
could reduce the energy consumption in the preparation of iron
loaded carbon from agricultural waste. In the present work, PC,
the by-product of bio oil production by pyrolysis, was used to
produce Fe/activated carbon by an improved method.
Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of
Chemistry, Sichuan University, Chengdu, Sichuan, 610064, China. E-mail: gchem@
scu.edu.cn; chwehu@mail.sc.cninfo.net; changweihu@scu.edu.cn; Fax: +86-28-
8
5411105; Tel: +86-28-85411105
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at 40 kV and 25 mA. The diffracted intensity was measured over
the 2q ranged from 5 to 80 .
2
. Experimental
ꢀ
ꢀ
2.1 Preparation of Fe/activated carbon
SEM was used to observe the morphological features of the
Pyrolytic carbon (PC) was produced from rice husk in a uidized samples. The experiment was performed on INSPECTF with an
ꢀ
bed reactor, where the rice husk was exposed to 475 C for less acceleration voltage of 20 kV. Samples were coated with gold
2
than 2 s to get bio-oil for the purpose of energy material. The before measurements.
PC with a particle size of 40–80 mesh was washed by distilled
The X-ray photoelectron spectroscopy (XPS) was performed
water and then dried at 110 C for 24 h. Then 30.00 g PC was on a AXIS Ultra DLD (KRATOS) high performance electron
mixed with 200 mL 3 M NaOH aqueous solution and the spectrometer with Al Ka radiation (1486.6 eV). The Al Ka X-ray
ꢀ
ꢀ
mixture was kept at 100 C for 5 h. The mixture was cooled to source was operated at 25 W, and the binding energy (BE) was
room temperature and ltered, and the ltrate of sodium sili- calibrated using the C 1s peak at 284.5 eV. A Shirley background
16
cate solution could be used to produce silica. The residue was was substrated from all spectra. The peak tting was performed
washed by water to reach a constant pH of about 7 and dried at with 80/20 Lorentz–Gauss function. The soware XPSPEAK41
ꢀ
17
1
10 C for 24 h, and was named as PC-b.
.00 g PC-b was immersed in 70 mL iron solution containing
n (the controlled amount) mmol Fe(NO ) $9H O. The mixture
was employed to t the peaks of Fe 2p, C 1s and O 1s.
5
3
3
2
2.3 Activity test
was stirred under ultrasonic instrument within 0.5 h. The
impregnation was maintained at 70 C till the complete evap-
oration of water and then it was dried at 110 C for 24 h. The
dried material (C–Fe) were introduced into a tube reactor and
heated from room temperature to the nal activation temper-
ꢀ
The hydroxylation of phenol was carried out in 50 mL two-
necked round-bottom ask equipped with reux condenser
and temperature-controlled water bath. The reaction was carried
out under the following typical conditions: phenol, 1.00 g; H O ,
1.00 mL; water, 10.00 mL; reaction time, 0.5–15 min; reaction
temperature, 30 C. The pH value of the reaction system was
ꢀ
2
2
ꢁ1
ature under nitrogen ow of 60 mL min . The samples were
kept at the nal temperature for 1.5 h before cooling down to
room temperature. The activation temperatures used were
ꢀ
adjusted to about 3 by adding acetic acid. Aer reaction, the
mixture was taken out and ltered. Finally, the liquid phase was
analyzed by high performance liquid chromatography (Waters
1525p) equipped with a 2847 ultraviolet detector at 277 and 254
nm using a reverse phase C18 column. The main products were
quantied using o-cresol as internal standard, while acetonitrile/
water (v/v ¼ 1 : 9) was used as the mobile phase. The amount of
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
4
00 C, 500 C, 550 C, 650 C, 750 C, and 850 C. The resultant
samples were denoted as C–Fe–y–z (y was the activation
temperature, C, z was iron content mmol g ).
ꢀ
ꢁ1
2
.2 Characterization of the samples
dihydroxybenzenes (DHB) is the sum of n
HQ CAT
and n . The
conversion of phenol (Xph), the yield of dihydroxybenzenes
(YDHB), the selectivity to dihydroxybenzenes (SDHB), the selec-
tivity to hydroquinone (SHQ), the selectivity to catechol (SCAT),
and the selectivity to benzoquinone (SBQ) are dened as follows:
PC and iron impregnated carbon (C–Fe) were heated at a rate of
ꢀ
ꢁ1
ꢀ
5
C min , from room temperature to 800 C in nitrogen ow.
The evolved gas was analyzed by mass spectrometry (MS). CO
2
arising from the oxidation of the sample was monitored by
recording the m/z ¼ 44 signal. NO arising from the reactive
0
ph
0
Xph ¼ (n ꢁ n )/n
ph ph
3 3
decomposition of Fe(NO ) was monitored by recording the
m/z ¼ 30 signal. H O and NO were monitored by recording the
2
2
0
Y
DHB ¼ (nHQ + nCAT)/nph
m/z ¼ 18 signal and 46 signal, respectively.
IRIS Advantage ICP-AES was used to analyze the amount of
metal elements contained in the PC and Fe/activated carbon
0
ph
SDHB ¼ (nHQ + nCAT)/(n ꢁ n )
ph
ꢀ
0
ph
samples. 0.20 g sample was burnt up to ash at 800 C for 3 h in
SCAT ¼ nCAT/(n ꢁ n )
ph
air. The obtained ash was dissolved in 1 : 1 hydrochloric acid
solution and further diluted to 100 mL by distilled water. Thus,
0
S
HQ ¼ nHQ/(nph ꢁ nph
)
9
the metal content could be obtained by analyzing the solution.
0
ph
SBQ ¼ nBQ/(n ꢁ n )
The specic surface area and pore size distributions of the
ph
ꢀ
samples were measured by N adsorption at ꢁ196 C using a
2
0
where nph and nPh denote the initial and nal amounts (moles)
of phenol, respectively, while nHQ, nCAT, and nBQ denote the
produced amounts (moles) of hydroquinone, catechol, and p-
benzoquinone, respectively.
Micromeritics TriStar 3020 instrument. The BET surface area
(
S
BET) was calculated from N
2
adsorption isotherms by using the
BET equation. Micropore volumes (Vmic), micropore surface area
mic) and external surface area (Sext) were calculated using the t-
16
(S
plot method. The average pore diameter was estimated from the
9
surface area and pore volume. Prior to gas adsorption measure-
3
. Results and discussion
ꢀ
ꢀ
ments, the samples were degassed at 120 C for 1.5 h and 300 C
ꢁ
2
3.1 Characterization of the samples
for 2.5 h in vacuum condition until a pressure of less than 10 Pa.
The XRD measurement was carried out on a DANDONG
3.1.1 TPD-MS of PC and C–Fe. PC and C–Fe were
ꢀ
FANGYUAN DX-1000 instrument with Cu Ka radiation, operated heated from room temperature to 800 C under nitrogen ow of
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ꢁ
1
1
0 mL min . The gas evolved in the heating process was ana- that the decomposition of nitrate iron on activated carbon
ꢀ
lysed by mass spectra, and the result was shown in the Fig. 1. In started to occur at about 140 C in air atmosphere, emitting
the heating process of PC, H O and CO were released. CO was NO2. The different result in the nitrogen oxides released might
produced at 240–700 C, while H O was released at 30–200 C. be caused by the atmosphere and substrate used. In previous
10
2
2
2
ꢀ
ꢀ
2
In the heating process of C–Fe, H
2
O, CO
2
and NO were released. work, commercially available AC was used and the decompo-
ꢀ
NO was released at 120–350 C. At the same times, CO
2
started sition of Fe(NO
3
)
3
occurred in air atmosphere. No simultaneous
2
had been observed. In the present work,
ꢀ
to emit at about 120 C for C–Fe, while for PC it started at formation of CO
ꢀ
2
40 C. It was indicated that Fe species interacted with carbon Fe(NO
3
)
3
on the PC reacted with carbon and produced NO and
and this kind of interaction made the oxidation of carbon start CO
2
at the same time in nitrogen atmosphere. The amount of
at much lower temperature. In the heating process of C–Fe, CO and NO released from C–Fe could be obtained by the peak
2
Fe(NO3)3 could oxidize carbon and change the properties of area. The CO2 and NO released were 0.481 mmol and 0.273
activated carbon, creating pore in carbon and generating CO2, mmol, respectively. According to the ratio of Fe(NO ) added,
3 3
ꢁ
Fe
2
O
3
and NO, that is, the reaction (1) occurred. It was reported the amount of NO
3
in C–Fe (0.10) was 0.30 mmol, while the
amount of NO from the decomposition was 0.273 mmol. These
ꢁ
data indicated that NO
3
decomposed completely in the heat-
ꢀ
ing process. Further more, C–Fe was heated from 105 C to 700
C by 5 C min under nitrogen ow, and the process was also
ꢀ
ꢀ
ꢁ1
analyzed by TG. The result of TG showed that C–Fe had 30.0%
weight loss. Meanwhile the data were consistent with the sum of
CO and NO (29.4%).
2
4
Fe(NO
3
)
3
(s) + 9C (s) ¼ ¼ 2Fe
2
O
3
(s) + 12NO (g) + 9CO
2
(g)
1)
(
3.1.2 The content of Fe and BET. The actual iron contents
of the prepared Fe/activated carbon samples were determined
by ICP, and the surface area and pore size distribution were
determined by BET. The results were listed in Table 1. It was
shown that the actual iron content on the samples increased
ꢁ1
from 0.15 to 2.0 mmol g . The N
2
adsorption isotherms shown
in Fig. 2 corresponded to typical type I in the BDDT classica-
18
tion, which indicated that all the samples were micropore
materials. The hysteresis loops in the nitrogen isotherm rep-
resented the existence of some mesopores, and the hysteresis
loops of parallel adsorption and desorption branches were
19
indicative of slit-shaped pores. The average pore diameters
were between 2.03 nm and 2.46 nm, and the pore size distri-
butions focused on 1.0–1.8 nm mostly, indicative of its micro-
pores character.
Fe species beneted the production of pores on the carbon
when the Fe content was low. With the increase of Fe content,
the specic surface area of carbon increased rst and
then decreased, and the maximum specic surface area
2
ꢁ1
ꢁ1
of 527 m
0
g
was obtained when the Fe content was
Fig. 1 Analysis of the gas in the heating process of PC and C–Fe.
.39 mmol g . With the increase of the Fe content, SBET, Smic
,
Table 1 BET of PC and different Fe content Fe/activated carbon samples
ꢁ1
2
ꢁ1
2
ꢁ1
3
ꢁ1
Sample
Fe content (mmol g
)
A
BET (m g
)
A
mic (m g
)
V
mic (cm g
)
Average pore width ( A˚ )
PC
0
96
427
527
498
480
434
352
77
350
392
396
393
360
234
0.031
0.141
0.160
0.161
0.159
0.146
0.096
24.6
20.9
23.4
21.3
21.3
20.3
35.4
C–Fe-750-0.15
C–Fe-750-0.39
C–Fe-750-0.72
C–Fe-750-1.0
C–Fe-750-1.3
C–Fe-750-2.0
0.15
0.39
0.72
1.0
1.3
2.0
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produced Fe
3
O
4
and CO
2
, which was in accordance with the
result of mass spectra. With further increase in temperature,
the pore was destroyed and it led to the decrease of surface area
and expansion of average pore width. High temperature will
destroy the structure of carbon, and is not conductive to the
16
pore formation. It is shown that the formation of activated
carbon and the loading of Fe is successfully combined in one
step, achieving supported iron catalysts with high specic
surface area.
3.1.3 XRD. Fig. 3a displays the XRD patterns of the Fe/
activated carbon at different activated temperatures. XRD
peaks of iron species could not be detected until the calcination
ꢀ
temperature increased up to 400 C, which indicated that Fe
species dispersed highly on activated carbon when calcined at
ꢀ
20,21
lower temperatures (#400 C).
.0, C–Fe-110-1.0, and C–Fe-400-1.0 were quite disperse, which
The XRD patterns of C–Fe-70-
1
was possibly caused by the low content of Fe. On C–Fe-450-1.0,
C–Fe-500-1.0 and C–Fe-550-1.0, obvious diffraction peaks were
ꢀ
detected at 30.0, 35.5, 44.0 and 57.5 . By comparison with the 2q
value of the C–Fe-550-1.0 and the patterns of standard magne-
tite (JCPDS Card no.: 19-629), it can be found that all peaks
correspond to magnetite Fe
acteristic peaks belonging to other iron oxides (Fe
present. However, it had no typical diffraction peaks of Fe(0) at
3
O
4
were present, and weak char-
2 3
O
) were also
ꢀ
11
4
5.0 and 65.0 in the XRD patterns. It implied that the
ꢀ
reduction reaction (2) occurred at 400–550 C, and carbon
reacted with Fe O leading to augment of pores. The result
2
3
accorded with the BET data, that is, the surface area of C–Fe-
00-1.0 increased a lot compared to C–Fe-400-1.0. Fig. 3b indi-
2
Fig. 2 Adsorption–desorption isotherms of N and pore size distri-
butions for PC and iron-impregnated activated carbon samples.
5
cated that C–Fe-650-1.0 had the peaks of Fe
activated above 750 C exhibited the peaks of metal iron. In the
heating process, ferric nitrate took part in a series of reactions
3
O
4
. The samples
ꢀ
Sext, and Vmic all decreased. The results were most probably due
to the consequence of pore blockage aer deposition of iron on
22,23
3+
and different iron oxides were produced.
Fe is partially
2+
reduced to Fe and Fe O is produced. Fe O is further reduced
3
4
3 4
7
the mouth of small pores.
ꢀ
to Fe at 650–750 C by carbon, that is, the reactions (3) and (4)
The BET result of different temperature activated Fe/
activated carbon samples were listed in Table 2. With the
increase of the activation temperature, the surface area of
activated carbon samples increased rstly, reached the
occurred.
6
Fe
2
O
3
(s) + C (s) ¼ ¼ 4Fe
3
O
4
(s) + CO
2
(g)
(2)
(3)
(4)
ꢀ
ꢀ
maximum at 750 C, and then decreased. Below 750 C, the
2Fe O (s) + 3C (s) ¼ ¼ 4Fe (s) + 3CO (g)
2
3
2
increase of temperature is benecial to the formation of pores
in the carbon. Especially, when the temperature increased from
Fe
3
O
4
(s) + 2C (s) ¼ ¼ 3Fe (s) + 2CO
2
(g)
ꢀ
4
00 to 500 C, the surface area of Fe/activated carbon increased
2
ꢁ1
obviously from 255 to 358 m g . This might be caused by the
3
+
The XRD result of the samples with different iron content
fact that Fe was oxidable, while carbon was reducible over this
temperature range. The carbon reacted with Fe and
ꢀ
activated at 750 C was presented in Fig. 3c. The peaks of both
2 3
O
Table 2 BET of different activated temperature Fe/activated carbon samples
ꢀ
2
ꢁ1
2
ꢁ1
3
ꢁ1
Average pore width ( A˚ )
Sample
T ( C)
A
BET (m g
)
A
mic (m g
)
V
mic (cm g
)
C–Fe-400-1.0
C–Fe-500-1.0
C–Fe-550-1.0
C–Fe-650-1.0
C–Fe-750-1.0
C–Fe-850-1.0
400
500
550
650
750
850
255
358
388
425
480
325
180
276
295
352
393
196
0.073
0.112
0.120
0.142
0.159
0.081
26.3
22.7
24.0
20.5
21.3
34.1
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3.1.4 XPS. The surface composition and chemical state of
Fe in the iron impregnated samples were analyzed by XPS and
presented in Fig. 4. All the binding energies were referenced
to C 1s at 284.6 eV. As expected, distinct C 1s, O 1s, and Fe 2p
peaks are observed in the survey scan spectrum for the
samples. There was no peak of iron compound on the PC
without ferric nitrate impregnated. On the Fe/activated
carbon samples, the typical Fe 2p XPS narrow scan spec-
trum present two main peaks at 711.2 and 725.0 eV corre-
sponding to the spin-orbit split doublet of Fe 2p3/2 and Fe
2p1/2, respectively. The peaks were unsymmetrical, and they
were divided into peaks to analyze the ingredient. The peaks
at the binding energy of 709.0–710.0 eV, 710.0–712.0 eV,
II
7
24.0–725.0 eV and 726.0–727.0 eV were ascribed to Fe 2p3/2,
III
II
III
Fe 2p3/2, Fe 2p1/2, and Fe 2p1/2. In addition, a satellite
peak around 719.0 eV conrmed the presence of Fe species
in the surface of all the samples. The shoulder peak at 714.6
III
7
eV might be ascribed to iron ions complexed with electro-
9
negative surface ligands. These could be identied as
2
0,24
Fe
3
O
4
.
The C–Fe-110-1.0 was presented in Fig. 4b, and
III
only the Fe 2p3/2 peaks were observed. It implied that the
sample was just dried at 110 C. The O 1s binding energy of
ꢀ
5
32.5 and 533.7 eV could be assigned to oxygen in SiO2
impurity (Si]O) and phenol oxygen (C–O). As shown in
Fig. 5, the PC has the peak of SiO . The peaks of PC-b and
C–Fe-750-1.0 were moving to right clearly, and it indicated
that the SiO in the PC was removed by NaOH solution
2
2
effectively. The O 1s peak at 530.3 eV observed on the
C–Fe-750-1.0 could be assigned to the Fe–O bond.
3.1.5 SEM. Scanning electron micrography (SEM) was
used to perform the morphological examination of the PC
and C–Fe-750-1.0 and the result was shown in the Fig. 6. The
surface of PC was smooth and there was a lot of small solid
particles on C–Fe-750-1.0. It was indicated that iron was
loaded on the carbon successfully. It can be seen that the iron
oxide particles distributed on the porous structure and
covered the surface of activated carbon in the impregnation
process.
3
.1.6 Magnetism of PC and iron impregnated carbon. The
magnetic characterization by a vibrating sample magnetometer
LakeShore 7410) was depicted in Fig. 7. From the plot of
(
magnetization (M) and magnetic eld, the PC had very weak
hysteresis. It revealed that PC was not a magnetic material and
ꢁ
1
the average saturation magnetization was less than 0.10 emu g
The saturation magnetization of C–Fe-400-1.0 was 0.97 emu g
.
ꢁ1
.
The results indicated that the sample gained magnetic property
as a result of the loading of Fe species. The magnetic property
was contributed by the presence of Fe O . The saturation
3
4
ꢁ1
Fig. 3 XRD of the iron-impregnated activated carbon.
magnetization of C–Fe-550-1.0 and C–Fe-750-1.0 were 2.98 emu g
and 5.73 emu g , respectively. It indicated that magnetic
substance was produced on the carbon with increased activa-
ꢁ
1
2 3 3 4
Fe O and Fe O were observed, and with the increase of the tion temperature.
iron content, the diffraction peaks for metal iron appeared. As
shown in Fig. 3c, on the C–Fe-750-2.0, obvious diffraction peaks
corresponding to metal iron were detected at 45.0 and 65.0 .
ꢀ
3.2 Catalytic activity studies
With the increase of activated temperature, the peaks of iron
metal were more obvious.
3.2.1 Phenol hydroxylation reaction. The results were pre-
sented in Table 3. All the iron-containing catalysts showed
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Fig. 4 Fe 2p XPS spectra analysis for iron-impregnated activated carbon samples.
obvious activity for phenol hydroxylation. The yield of dihydroxy
benzenes (YDHB) was low without catalyst at different tempera-
ture. The Xph increased with increasing temperature, but the
Xph and YDHB were low without catalysis. Especially, the reaction
did not produce HQ under ambient conditions. It was clear that
iron was responsible for the activity. As shown in Table 4, with
the content of iron on the catalyst increased, the YDHB increased
and then decreased. The C–Fe-750-1.0 exhibits the best activity
for the phenol hydroxylation. In Table 5, the effect of the
amount of catalyst for phenol hydroxylation was researched.
The optimum amount of catalyst was 0.1 g and YDHB could reach
34.0% with SDHB of 65.8%. The effect of reaction time for phenol
hydroxylation was present in Table 6. The C–Fe-750-1.0
Fig. 5 O 1s XPS spectra analysis for PC, PC-b and C–Fe-750-1.0.
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exhibited high phenol hydroxylation activity and YDHB could
reach 37.0% within 6 min. With the prolongation of time, YDHB
did not increase further.
In the phenol hydroxylation reaction, an induction
period of 5ꢁ120 min was previously mentioned in the
literature. It was reported that adding a small amount of
CH
period.
3
COOH could shorten and eliminate the induction
3
,25,26
Many studies have shown that the phenol
hydroxylation needed a reaction time of 1–3 h, with the
phenol conversion of 40–50%, and the selectivity of
1
0,27
dihydroxybenzenes is within 70.6–90%.
C–Fe-750-1.0
could shorten the reaction time greatly, because of
different valence state iron species were produced on the
carbon.
It is reported that Fe
performance than Fe
3 4
O species present higher catalytic
28
2
O
3
for the activation of H
2
O
2
.
The
preparation of this catalyst from agricultural waste combined
activation process and iron loading process to one step. It
reduced the energy consumption and produced value added
samples.
Fig. 6 SEM of the PC and C–Fe-750-1.0.
3.2.2 Leaching analysis. The iron content of C–Fe-750-1.0
aer reaction testing and the iron leaching in the reaction
solution were analyzed by ICP. The iron content remained on
the used catalyst (C–Fe-1) was 0.027 mmol, and nearly one
half of iron on activated carbon leached. 0.05 g C–Fe-750-1.0
and 10 mL solvent (H O and acetic acid) were carried out in 50
2
ꢀ
mL two-necked round-bottom ask at 30 C. Then, the
mixture was taken out and ltered. The carbon (leached-C)
obtained aer removal of all leachable sites, as well as the
2
9
leachates.
The leached-C and leachate catalyzed the
hydroxylation of phenol, in the same conditions, respectively.
As shown in Table 7, the reaction activity was mainly inu-
enced by leached-C, that is, the heterogeneous catalyst
contributed higher activity than the leached species in the
solution.
Fig. 7 Magnetic hysteresis cycles for the PC and iron impregnated carbon.
Table 3 Effect of temperature for phenol hydroxylation without catalysis
ꢀ
Catalyst
H
2
O
2
(mL)
T ( C)
t (min)
X
ph (%)
YDHB (%)
SDHB (%)
SCAT (%)
SHQ (%)
SBQ (%)
—
—
—
—
—
1
1
1
1
1
30
40
50
60
70
60
60
60
60
60
5.6
5.7
5.4
6.3
8.0
0.0
0.1
0.1
0.1
0.1
0.7
1.1
1.0
1.8
1.3
0.7
1.1
1.0
1.8
1.3
0.0
0.0
0.0
0.0
0.0
0.1
0.4
0.6
1.1
0.7
Table 4 Effect of iron content of catalysis for phenol hydroxylation
ꢀ
Catalyst
Mc (g)
H
2
O
2
(mL)
T ( C)
t (min)
Xph (%)
YDHB (%)
SDHB (%)
SCAT (%)
SHQ (%)
SBQ (%)
C–Fe-750-0.39
C–Fe-750-0.72
C–Fe-750-1.0
C–Fe-750-1.3
C–Fe-750-2.0
0.05
0.05
0.05
0.05
0.05
1
1
1
1
1
30
30
30
30
30
1
1
1
1
1
5.5
8.0
52.7
50.9
44.1
0.1
3.8
31.4
31.3
25.4
2.2
2.2
0.0
18.1
26.2
30.4
24.3
0.4
7.2
0.1
0.0
0.1
47.8
59.6
61.5
57.5
29.7
33.4
31.1
33.1
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Table 5 Effect of amount of catalysis for phenol hydroxylation
ꢀ
Catalyst
Mc (g)
H
2
O
2
(mL)
T ( C)
t (min)
Xph (%)
YDHB (%)
SDHB (%)
SCAT (%)
SHQ (%)
SBQ (%)
C–Fe-750-1.0
C–Fe-750-1.0
C–Fe-750-1.0
C–Fe-750-1.0
C–Fe-750-1.0
0.02
0.05
0.10
0.15
0.20
1
1
1
1
1
30
30
30
30
30
1
1
1
1
1
42.8
52.7
50.6
50.6
49.7
27.5
31.4
33.3
30.2
29.2
64.3
59.6
65.8
59.7
58.7
39.4
33.4
33.9
31.3
30.2
25.0
26.2
31.9
28.3
28.6
1.3
0.1
0.1
0.0
0.1
Table 6 Effect of reaction time for phenol hydroxylation
ꢀ
Catalyst
Mc/g
H
2
O
2
(mL)
T ( C)
t (min)
X
ph (%)
YDHB (%)
SDHB (%)
SCAT (%)
SHQ (%)
SBQ (%)
C–Fe-750-1.0
C–Fe-750-1.0
C–Fe-750-1.0
C–Fe-750-1.0
C–Fe-750-1.0
0.05
0.05
0.05
0.05
0.05
1
1
1
1
1
30
30
30
30
30
0.5
1
3
6
15
29.0
16.8
57.8
36.0
33.4
33.7
37.8
36.6
21.8
26.2
24.9
28.0
28.2
0.4
0.1
0.1
0.3
0.1
52.7
55.0
56.2
56.4
31.4
32.2
37.0
36.5
59.6
58.5
65.7
64.7
Table 7 Leaching analysis of C–Fe-750-1.0 in the hydroxylation of phenol
ꢀ
Catalyst
H
2
O
2
(mL)
T ( C)
t (min)
Xph (%)
YDHB (%)
SDHB (%)
SCAT (%)
SHQ (%)
SBQ (%)
Leachate
Leached-C
1
1
30
30
6
6
17.6
26.4
10.5
16.8
59.7
63.6
41.2
42.2
18.5
21.4
12.1
6.8
2
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Fe(NO ) , and generated CO , Fe O and NO below 400 C.
3
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Fe O dispersed on activated carbon was formed when iron
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